This section is intended to provide a background or context to the invention disclosed below. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise explicitly indicated herein, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section. Acronyms used in this application or the drawings are defined below, prior to the claims.
As an overview, activities in the EU-funded project METIS as well as in many other research groups promise large potential gains for systems deploying massive MIMO. The main objective of the METIS project is to lay the foundation of 5G, the next generation mobile and wireless communications systems. MIMO uses multiple antennas to communicate with UEs. Massive MIMO for 5G is currently under the assumption of 100 or more antenna elements, and spectral efficiencies of several tens of bit/s/Hz/cell (e.g., bit or bits per Hertz per cell) have been reported under ideal conditions compared to about 3 bit/s/Hz/cell for a 4×2 MIMO case 1 system in 3GPP (see 3GPP TR 36.819 V11.0.0 (2011-09), table 7.2.1.2-5). Achievable gains in real world conditions are for further study, but it is obvious that having low cost massive MIMO arrays will be of great benefit.
In more detail, the above-mentioned large performance gains are the result of strong multi-user MIMO (MU MIMO) transmission, i.e., the spatial multiplexing of, e.g., ten or more users simultaneously in one time-frequency resource block combined with strong beamforming gains. Beamforming gains are achieved by providing over-provisioning of antenna elements. In the case of a factor of ten times more antennas than served users and greater than ten served UEs, the overall number of antenna elements will be in the order of 100 or more.
Today's LTE systems have as baseline two antenna ports being extendable up to 8×8 MIMO, which is rarely deployed. So there are so far only few RF frontends needed per sector. These RF frontends are currently complex and bulky devices, which contribute a significant part to the overall costs of an eNB.
Straight-forward implementation of large arrays based on one RF frontend per antenna element will lead to exploding costs, power consumption and size of the overall system. See for example the current status of active antennas, where RF frontends per active antenna element are so far in the order of 0.2×0.2×0.1 m3.
For active antennas, there are currently projects running to reduce the size to about 0.1×0.1×0.05 m3, which is still far from the ideally intended size of future massive MIMO antenna arrays. For example, the space for the RF frontends for a 256 element antenna array would be in the order of [0.1×0.1 m2]×256=2.45 m2 leading to an overall volume of 2.45*0.05 m3=0.1225 m3. In terms of feet, this is about 14.1 ft3.
The typical ideal vision for a future massive MIMO array is for example a flat panel placed at walls with an array of 16×16=256 antennas and a single low cost active device or chip per antenna element containing the full RE chain including the power amplifier (PA) as well as filters and the like. Part of the problem is the relatively high number of complex components per RF chain such as broadband high resolution analog digital converters (ADC and DACs), highly linear power amplifiers plus their control circuits for linearization with large headroom, bulky filters with strong out of band suppression (for example ceramic filters), and the like.
It would be beneficial to reduce the costs for the implementation of massive MIMO arrays, which is a real challenge taking the high number of antenna elements into account.